1. Field of the Invention
The present invention relates to steered arc coating processes and apparatus using thick cathodes or targets.
2. Description of the Prior Art
U.S. Pat. No. 4,673,477 discloses a controlled arc coating process and apparatus for coating of materials. The patent teaches the control of an arc path by utilizing a magnetic field positioned in a known relationship with respect to the cathode for defining a closed arc track along which the arc will continuously move.
U.S. Pat. No. 4,563,262 issued to Sablev et al. discloses a method of depositing alloys using a composite target. In this process a random or unsteered arc is employed acting on a composite target of a particular configuration.
U.S. patent application Ser. No. 848,720, filed Apr. 4, 1986, to Ramalingam et al. discloses a steered arc coating process and apparatus where the residents time of the arc is controlled in different parts of a solid composite cathode to permit depositing alloys on a substrate. Mechanical linkages control the arc path by moving the magnet that defines the arc path, and geometric proportioning of the composite cathode as well as arc operating parameters such as current provide the basis for control of a deposited alloy composition.
While the system works well with mechanical linkages, improvements in control of the arc path, and tight arc steering are necessary to produce macro particle free coatings when using thick cathodes or targets so an adequate supply of coating material is available. When the process and apparatus disclosed in the Ramalingam Patent 4,673,477 are used to produce macro particle free deposits, cathode thicknesses are usually restricted to about 20 millimeters (about 13/16 inch) or less. The Ramalingam et al. patent application Ser. No. 848,720, filed Apr. 4, 1986, and the Sablev et al. U.S. Pat. No. 4,563,262 produce coatings on a substrate where the composition change is abrupt at the substrate/deposited film interface. These same prior art disclosures can be used to produce multi-layer coatings, but more than one arc source is necessary to do so.
In any coating it is desirable, and is all but necessary, to have uniformly small grain or crystallite sizes, and thus depositing of macro particles is unsatisfactory, and produces coatings that are not within desired tolerances.
The present invention provides the necessary arc steering for producing macro particle free coatings with thick targets or cathodes, and, when desired, multi-layer coatings using a thick composite cathode.